Generally, integrated circuits and electrical components may be designed based on operational margins to allow them to operate within specific operational parameters without causing permanent damage to their internal circuitry. For example, the maximum operating voltage of a design integrated circuit may be set based on the electrical characteristics such as threshold voltage of the transistors in the integrated circuit. However, the electrical characteristics of the transistors can be affected by various reliability mechanisms (also referred to as aging effect, or transistor aging) such as bias temperature instability (BTI), hot carrier injection (HCI), and time dependent dielectric breakdown (TDDB). These mechanisms can lead to transistor aging (i.e., changes in transistor electrical characteristics), resulting in possible performance degradation or failure of the integrated circuit.
As metal-oxide-semiconductor field-effect transistor (MOSFET) technology is scaled down, the variations caused by transistor aging can have an increasing effect on the design and performance of the integrated circuit. At short gate lengths (e.g. transistor gate lengths shorter than 50 nm), even relatively small variations caused by transistor aging can result in reduced operating speed for the integrated circuit, as well as increased failures at low operating voltages.
Typically, the performance degradation due to transistor aging can progressively increase as the integrated circuit is operated during its lifetime. For example, transistor aging caused by bias temperature instability (BTI) can lead to increased transistor threshold voltage. BTI can lead to degradation of both PMOS and NMOS transistors.
Generally, the degradations caused by BTI can be divided into two types—static BTI and dynamic BTI. Static BTI results from the application of a constant voltage stress, i.e., a DC bias voltage (typically a negative bias for PMOS and positive bias for NMOS) to the gate terminal of the transistor while also applying a lower DC bias voltage to the source and drain terminals of the transistor. Static BTI can degrade the gate oxide of a transistor (e.g., by trapping charges in the gate oxide), which leads to an increased threshold voltage and reduced drive current Idsat. Dynamic BTI (i.e., frequency dependent aging) results from the application of a switching waveform (e.g, a clock waveform or an AC signal) to either the gate, source, or drain terminals of a transistor. Such increases in threshold voltage due to transistor aging can reduce the voltage overdrive and therefore, degrade the circuit stability and operating margin of the integrated circuit. Thus, both static and dynamic BTI can reduce the performance and yield of the integrated circuit.